1. Field of the Invention
The present invention relates to an optical encoder and a decoder, which are easily controlled, fully tunable, and have low amount of energy loss during encoding or decoding. In particular, the apparatuses according to present invention have simple structures, and can be integrated on a photonic integrated circuit so that the reliability is increased and the cost of production decreased.
2. Description of the Prior Art
Fiber optic code division multiple access networks can provide bursty access communication channels. With the use of all optical signal processing, a data capture rate of hundreds of giga bytes per second is obtained. In addition, the spreading sequence codes having adventages of sharp autocorrelation function, small crosscorrelation, easily encoded and decoded, and having low amount of energy loss during encoding or decoding are desired. Accordingly, it is important to find codes that have the above-mentioned characteristics in order to encode and decode effectively.
Although several kinds of optical encoders or decoders have been developed over the past years, there are still problems remaining. For example, some arrangements use a large number of fiber optical delay lines, so manufacturing the devices becomes difficult. Moreover, there is energy loss during encoding or decoding, and the devices are not perfectly tuned. In other arrangements, the encoder or decoder can be perfectly tuned. However, the method of controlling the device is complicated, and the speed of the controlling signals is not high enough to effectively operate the device.
The spreading sequence technology for application in the fiber optical communication and methods of encoding and decoding prime codes are described in an article by P. R. Prucnal, xe2x80x9cIEEE J. Lightwave Technol.,xe2x80x9d vol.4, no.5, pp.547-554, 1986. If the prime number is p, the encoder or decoder operates when p fiber optical delay lines having constant lengths is provided, respectively. However, the encoder or decoder mentioned above can merely encode or decode identical item codes, and the remaining optical power after the encoding and decoding process is 1/p2 of the original amount of power.
Quasi-prime codes produced by fiber lattice of an encoder or a decoder are presented in an article by A. S. Holmes, xe2x80x9cIEEE J. Lightwave Technol.,xe2x80x9d vol.10, no.2, pp.279-286, 1992. The fiber lattice of an encoder comprises a plurality of Electro-Optic (EO) couplers and fiber optical delay lines, wherein each Electro-Optic coupler has two input and two output terminals (2xc3x972 EO coupler hereinafter), and these 2xc3x972 EO coupler are connected in series. The encoder is tunable; however, the numbers of the quasi-prime codes that the fiber lattice of the encoder can produce are less than that of the prime codes. Therefore, it is inconvenient to vary the code forms for processing. Additionally, this arrangement of the device can""t operate effectively, and the amount of energy loss during encoding or decoding is substantial.
In an article of xe2x80x9cIEEE Trans. Commun. vol.44, no.9, pp.1152xcx9c1162, 1996xe2x80x9d by W. C. Kwong, 2n prime codes and an encoder and a decoder are described. The numbers of the 2n prime codes produced by the encoder are less than that of the prime codes mentioned above. In spite of this, the arrangement of the encoder is simpler than that in A. S. Holmes paper because only about log2n 2xc3x972 EO couplers connected in series and fiber optical delay lines are needed, where n is the length of the 2n prime codes. However, the encoder must be able to process high-speed electric signals.
An advance is described in U.S. Pat. No. 4,159,418, 1979 by E. Marom, which discloses an optical encoder or a decoder comprising of multi-mode optical fibers and several optical couplers. Generally, the encoder or decoder can merely encode or decode identical item codes. Further, the amount of energy loss will increase significantly if the multi-mode optical fiber is substituted by single mode optical fiber. Therefore, this kind of optical encoder or decoder is not appropriately applied in the fiber optic code division multiple access networks.
Another advance is described in the U.S. Pat. No. 5,610,746, 1997 by E. R. Ranalli, which discloses a wide band optical source comprising a light emitting diode (LED) that emits incoherent light which is then focused by a coupling lens into a waveguide. In addition, a coupler comprising an amplifier and a plurality of feedback waveguides which are rendered transmissive or absorbing is also described in the patent. This device is tunable; however, the feedback waveguides make the arrangement of the device complicated, and the amount of energy loss during encoding or decoding is substantial.
Accordingly, the object of the present invention is to provide an optical encoder and a decoder that are easily controlled, fully tunable, and have low amount of energy loss during encoding or decoding. Another object of the present invention is to provide an optical encoder and a decoder that have simple structures, and can be integrated on a photonic integrated circuit so that the reliability is increased and the cost of production decreased.
To achieve the above-mentioned object, an optical encoder for outputting multiple-level codes is provided, comprising: an Electro-Optic (EO) coupler having a first and a second input terminal, and having a first and a second output terminal; and an optical recirculating loop which comprises an amplifier coupled between the second output terminal and the second input terminal, wherein the first input terminal receives laser pulses, and the EO coupler divides the received laser pulses into a first group and a second group according to a coupling ratio, then couples the first group of laser pulses (feedback laser pulses) to the optical recirculating loop by the second outputting terminal, so that the feedback laser pulses are amplified by the amplifier, and the second input terminal receives the amplified feedback laser pulses which are fed back to the second group of laser pulses by the EO coupler, and the first output terminal outputs multiple-level pulses.
Furthermore, to achieve the above-mentioned object, an optical decoder for outputting a bit value is provided, comprising: an Electro-Optic (EO) coupler having a first and a second input terminal, and having a first and a second output terminal; an optical recirculating loop which comprises an amplifier coupled between the second output terminal and the second input terminal, wherein the first input terminal receives multiple-level pulses, and the EO coupler divides the received multiple-level pulses into a first group and a second group according to a coupling ratio, then couples the first group of multiple-level pulses (feedback multiple-level pulses) to the optical recirculating loop by the second outputting terminal, so that the feedback multiple-level pulses are amplified by the amplifier, and the second input terminal receives the amplified feedback multiple-level pulses which are fed back to the second group of multiple-level pulses by the EO coupler, and the first output terminal outputs light waves; a light detector, coupled to the output input terminal of the EO coupler, transforming the optical energy of the light waves into an electrical current; and a bit determining device coupled to the light detector for outputting a bit value.